The spectra resulting from observations of unresolved galaxies undergoing vigorous star formation originates from the emission given off by a large collection of H II regions within each galaxy. This gives rise to powerful selection effects: the emission lines we observe come from clouds with the physical conditions necessary to optimally emit them. Current modeling of star forming galaxies assumes a small range of conditions typically seen in local H II regions. Using spectral synthesis simulations that cover a vast parameter space, we have compiled an atlas of starburst galaxy emission line equivalent widths ranging from the UV to IR in order to assist observers in understanding the conditions present within their galaxy sample. With our own galaxy sample that spans a large range of ionization, we used this atlas to fit emission line ratios much better than when using previous modeling methodologies, revealing that the distribution of ionizing flux within a star forming galaxy, instead of its metallicity, is the primary physical parameter controlling the ionization level.